1. Field of the Invention
This invention relates generally to an optical tracking apparatus for optically tracing a spiral track on a record disk, and more particularly, is directed to an optical tracking apparatus having particular utility in an audio PCM disk player, a video disk player or a video disk recorder.
2. Description of the Prior Art
Audio PCM information can be spirally recorded as numerous pits on a transparent disk by an optical audio PCM disk player, and then read out by an optical pickup device carried on an arm. An arm drive apparatus for a conventional player can be used for tracking with the pickup device. In the arm drive apparatus, the arm is supported at one end by an arm support which is rotatably supported by an arm shaft. Drive coils are fixed at both ends of the arm support and a magnetic field forming apparatus for interacting with the drive coils includes upper and lower ring yokes which are fixed to each other by a pair of permanent magnets. The drive coils are slidably fit on the upper ring yoke. Current flowing through the drive coils interacts with the magnetic field between the upper and lower ring yokes to generate an electromagnetic rotational torque for the arm.
In the above-described arm drive apparatus, the path for the magnetic field formed by the drive coils is constituted by the upper ring yoke which is a closed circuit. Accordingly, self-inductance of the drive coils is extremely so high so as to deteriorate the frequency response characteristic of the tracking servo system due to the integration effect of the drive coils. In other words, the tracking servo system cannot follow tracking errors of high frequency. Further, iron loss of the upper ring yoke is large, and accordingly, the drive efficiency of the drive coils is low. The drive circuits of the servo system are therefore complicated and expensive.